Uncertainty refers to situation when there is more than one possible outcome to decision and where the probability of each specific outcome is not known what is the reason behind of this ?

Assume that (i) setups need to be completed first; (ii) a setup can only start once the batch has arrived at the resource, and (iii) all flow units of a batch need to be processed at a resource before any of the units of the batch can be moved to the next resource. process step 1 molding 2 painting 3 dressing setup time 15 min. 30 min. no setup processing time 0.25 min./unit 0.15 min./unit 0.30 min./unit which batch size would minimize inventory without decreasing the process capacity?